This invention relates generally to adjusting the properties of devices such as surface acoustic wave (SAW) devices, and, more particularly to a system and method for adjusting the frequency of SAW devices by processing the surfaces of the devices by gas cluster ion beam (GCIB) irradiation.
SAW devices are used in a variety of applications, such as resonators for frequency generation oscillators, delay lines, pressure transducers, or as filters. Setting the frequency of SAW devices precisely for a specific resonator or filter application can be a difficult task, especially if numerous devices are required to be set to a specific frequency within a tight tolerance xcx9c100 PPM or less.
Generally, a SAW device comprises a pair of transducers, but sometimes more, with each transducer having a set of conductive members which is disposed on or recessed within an upper portion of a surface which supports surface acoustic wave propagation. As SAW devices find new applications, the requirements for precision in the frequency characteristics of the surface acoustic wave device increase. Accordingly, in many applications, it is now desired to have the center frequency of the device within xc2x11 ppm of the design frequency. Many factors contribute to deviations from the design center frequency of a SAW device including the fabrication techniques presently used to manufacture SAW devices. Typically, with present techniques, the after fabricated SAW device has an actual center frequency within about xc2x1100 ppm of the design frequency. Accordingly, the frequency characteristic of the fabricated devices must be modified either upwards or downwards in frequency to meet the design frequency. Typical SAW devices for commercial cellular telephone applications are made up of a set of interdigitated transducers (IDTs) deposited onto quartz substrates using conventional photolithography processes. The IDTs have been formed from a variety of metals including pure aluminum, copper-doped aluminum, titanium-doped aluminum, tantalum, or other metal or combination of metals. The variation in the photolithography definition from substrate to substrate, as well as from device to device on the same substrate can cause large frequency variations in a batch of SAW devices. Several techniques have been used to trim or adjust the frequency of these devices including reactive ion etching (RIE) to reduce the SAW device frequency and ion beam milling. For very tight tolerance, it is difficult to use RIE, and ion beam milling can cause significant damage to the single crystal quartz substrate leaving the device unusable for its intended application.
More specifically, SAW devices are typically formed on quartz single crystal substrates varying in size from 2 to 5 inches in diameter. A photomask is used to print patterns of multiple devices on each substrate. Several substrates are then put into a deposition system to deposit the IDTs. Typically the deposition will be carried out by electron beam evaporation. Variations in the evaporation process from run to run, or within a single run can yield variations in film thickness from substrate to substrate, or from batch to batch. As a result of the variations in the photomask, photolithography process, deposition process, and other processing variables, the frequencies of the resulting SAW devices can vary significantly, making the devices unusable for the intended application without some method of post-fabrication frequency modifying operation.
Several techniques are commonly employed in the art to change the frequency characteristics of a SAW device. One technique known as air-baking involves exposing the SAW device to air disposed at an elevated temperature for a limited period of time to produce an upshift in the center frequency of the device. The utility of air-baking is relatively limited, however, since air-baking has not proven to be a reproducible technique, and furthermore, the amount of frequency shift obtained during the air-baking process is extremely limited particularly at frequencies below 500 MHz.
A second method involves using etching techniques such as RIE. The reactive ion etching techniques involve sophisticated equipment, in which the SAW device is exposed to fluorine ions produced by an r.f. discharge. The fluorine ions selectively etch the surface wave propagation surface. The result of reactive ion etching is to trim down the center frequency of the SAW device. With reactive ion etching, frequency adjustment as much as xe2x88x92500 ppm may be obtained. Reactive ion etching, however, involves the use of relatively expensive and sophisticated equipment and, furthermore, the technique may involve relatively long etching times for devices in which a large frequency adjustment is necessary. Additionally RIE is extremely difficult to use for very tight tolerances. In addition, modifying the frequency has been performed by ion beam milling to remove metal from interdigitated transducers (IDTs) to increase the frequency. However, this technique can cause significant damage to the single-crystal quartz substrate, leaving the device unusable for its intended purpose.
Another technique known in the art is set forth in U.S. Pat. No. 4,243,960 by White et al. and in papers entitled xe2x80x9cFine Tuning of Narrow-Band SAW Devices using Dielectric Overlaysxe2x80x9d, 1977 Ultrasonic Symposium Proceedings, IEEE, pgs. 659-663 by Helmick et al. and xe2x80x9cObservation of Aging and Temperature Effects on Dielectric-Coated SAW Devices xe2x80x9d, 1978 Ultrasonics Symposium Proceedings, IEEE, pp. 580-585 by Helmick et al. This patent and these papers describe a technique in which a dielectric coating is provided on the surface wave propagation surface and in contact with the electrodes forming the interdigitated transducers, with the amount of frequency shift selected by controlling the thickness of the deposited coating. While the described technique produces frequency variations, these frequency variations come at the expense of a relatively large increase in the insertion loss of the device generally on the order of 1 db to 2 db, as well as, a relatively large increase in the so-called xe2x80x9cturnover temperaturexe2x80x9d of the piezoelectric material which supports the surface acoustic wave propagation.
Some materials that are commonly employed to support surface wave propagation, such as ST-cut and rotated ST-cuts of quartz, exhibit a parabolic surface wave velocity variation as a function of temperature. The maximum of this parabolic variation is referred to as the xe2x80x9cturnover temperaturexe2x80x9d. In many applications, the SAW device is designed to operate close to this temperature, particularly when the frequency stability of the SAW device is of critical importance. Large unpredictable variations in the turnover temperature place the device out of specification for such applications, since the cut of the substrate material is specified for its particular temperature dependent characteristic. Accordingly, the large shifts in the turnover temperature described in the above references make this technique impractical for use in many SAW device applications.
One form of SAW device includes a substrate having a surface for acoustic wave propagation at a predetermined surface acoustic wave velocity characteristic. There are a pair of IDTs coupled to the acoustic wave propagation surface. The two interdigitated transducers are on the substrate surface, and are spaced apart by a region of the acoustic wave propagation surface. A prior art method of adjusting the center frequency of such a SAW device is to adjust the surface wave velocity characteristic of the surface wave device by depositing a thin layer of a nonconducting elastic material, such as aluminum oxide or zinc sulfide onto a portion of the region separating the pair of interdigitated transducers to change the surface wave velocity characteristic of the surface wave device. This method is described in U.S. Pat. No. 4,757,283. A drawback of this technique is that the added (deposited) material has a tendency to undesirably change the relationship between surface wave velocity and ambient temperature for the SAW device.
A further technique for adjusting the characteristics of a SAW device involves depositing a pair of localized regions of frequency determining modifying material onto selected portions of a surface wave propagating surface, to provide localized regions on said surface where the odd order transverse mode has energy maxima. These regions change the acoustic properties and hence the velocity characteristics of the surface wave propagating surface in said regions. The frequency of the odd mode transverse wave is changed accordingly and is preferably changed to match that of the fundamental transverse propagating wave. This decreases the insertion loss of the device at the fundamental frequency and eliminates the odd mode transverse wave propagation characteristic within the operating frequency range of the resonator. Further, in the SAW device which includes an acoustically matched piezoelectric transparent cover disposed over the surface wave propagating surface having disposed thereon a trim pad is selectively removed to provide the localized regions and thus the localized alterations in the acoustic properties of the surface acoustic wave device as described in U.S. Pat. No. 4,933,588. While this is an effective and precise technique, it is an undesirably costly process for high volume commercial applications such as SAW devices for use in cellular telephones.
The use of a gas cluster ion beam (GCIB) for etching, cleaning, and smoothing of the surfaces of various materials is known in the art (See for example, U.S. Pat. No. 5,814,194, Deguchi, et al., xe2x80x9cSubstrate Surface Treatment Methodxe2x80x9d, 1998). Means for creation of and acceleration of such GCIB""s are also described in the Deguchi reference. It is also known (U.S. Pat. No. 5,459,326, Yamada, xe2x80x9cMethod for Surface Treatment with Extra-Low-Speed Ion Beamxe2x80x9d, 1995) that atoms in a cluster ion are not individually energetic enough (on the order of a few electron volts to a few tens of electron volts) to significantly penetrate a surface to cause the residual sub-surface damage typically associated with the other types of ion beam processing, including ion milling, in which individual ions may have energies on the order of hundreds or thousands of electron volts. Nevertheless, the cluster ions themselves can be made sufficiently energetic (some thousands of electron volts), to effectively etch, smooth or clean surfaces as shown by Yamada and Matsuo (in xe2x80x9cCluster ion beam processingxe2x80x9d, Matl. Science in Semiconductor Processing I, (1998) pp 27-41). It is also known (see Japanese laid open application 08127867 JP A, Akizuki et al., xe2x80x9cFormation of thin film by gas cluster ion beamxe2x80x9d) that GCIB formed from reactive gas source materials such as CO2, O2, N2, and other materials can be used to form thin films by irradiating a substrate with the GCIB to induce a chemical reaction of the GCIB materials with the substrate.
It is therefore an object of this invention to provide a system and method for effectively and precisely adjusting the properties of a device such as the properties of a SAW device including the characteristic frequencies of such a device.
It is a further object of this invention to provide a system and method to precisely adjust the properties of a SAW device so as to either increase or decrease the property value, including the characteristic frequencies of such device.
It is still another object of this invention to provide a system and method for adjusting the properties of a SAW device without significantly damaging or degrading the performance of the device or the IDTs.
The objects set forth above as well as further and other objects and advantages of the present invention are achieved by the embodiments of the invention described hereinbelow.
This invention comprises a system and method to precisely adjust the properties of devices, such as SAW devices. This includes, but is not limited to adjusting the characteristic frequency(ies) of SAW devices for specific resonator or filter applications. A gas cluster ion beam is generated and used to irradiate the surface of the SAW device, modifying the surface to change the properties, such as the center frequency of the SAW device in a controlled and predictable manner. The GCIB may be formed from an inert gas such as argon for removing small amounts of the surface materials by etching. The GCIB may also be formed from a reactive source gas such as O2 or N2, for example, for reacting with the surface to change its acoustical properties. The gas cluster ion dose is precisely controlled by monitoring the beam current during the exposure to provide a predetermined dose. The cluster ions are produced in a GCIB apparatus, having a selectable source gas, cluster size distribution, adjustable acceleration energy, and controllable dose delivered to the SAW device. A single SAW device or a substrate of multiple devices can have its properties adjusted by the use of this invention.